Conventional, high performance, supersonic military aircraft utilize a conventional afterburner and variable area, converging-diverging exhaust nozzle for providing high performance over a flight envelope ranging in speeds from subsonic to supersonic up to about Mach 2.5. The conventional exhaust nozzle includes primary exhaust flaps defining a straight sided converging channel, and secondary exhaust flaps extending downstream thereof to define a straight sided diverging channel. A throat of minimum flow area is disposed at the junction of the primary and secondary flaps and has a flow area conventionally designated A.sub.8. The outlet of the exhaust nozzle is disposed at the downstream ends of the secondary flaps and has an outlet flow area conventionally designated A.sub.9.
The ratio of the nozzle outlet area A.sub.9 over the throat area A.sub.8 varies during operation through the flight envelope by varying the positions of the primary and secondary flaps to accommodate variations in the conventionally known nozzle pressure ratio NPR defined as the total pressure of the combustion gases upstream of the converging channel divided by the static pressure downstream of the nozzle outlet i.e., ambient pressure. The NPR increases with increase in velocity of the aircraft, and for example may reach a value of about 30 at an aircraft speed of about Mach 2.5. Efficient expansion of combustion gases having high values of NPR requires a diverging channel having a relatively long length as compared to corresponding shorter length required for low NPR values.
Accordingly, the exhaust nozzle is designed for efficient expansion of the combustion gases at the highest expected NPR values, which necessarily results in a relatively long exhaust nozzle. The length of an exhaust nozzle directly affects its weight, complexity, and the amount of cooling air required for maintaining acceptable life of the exhaust nozzle, and, therefore, it is desirable to have exhaust nozzles as short as possible.
Advanced military aircraft are being considered for operation at speeds ranging from subsonic to relatively high supersonic greater than Mach 2.5 and up to about Mach 6. For example, the NPR value associated with Mach 6 operation of the aircraft is about 650 which is substantially higher than the NPR value of about 30 associated with flight at Mach 2.5. Accordingly, an exhaust nozzle utilized in such an aircraft must be designed for efficient expansion of combustion gases having such a relatively high NPR value. A conventional exhaust nozzle including conventional straight sided converging-diverging channels would be undesirably long for efficient expansion of the combustion gases at NPR values greater than 30 and up to about 650, and higher. The increased length would result in unacceptably high weight and complexity, and unacceptable cooling air requirements for such a nozzle. The cooling air typically is provided from compression in the aircraft engine, and any air not utilized for combustion, and utilized for cooling, for example, decreases the overall performance of the engine.